This invention relates to modified cis-1,4-polyisoprene rubber. More particularly, it is concerned with cis-1,4-polyisoprene rubber modified by incorporation of a side chain composed of a succinimide residue having in the N position a hydrocarbon group having a tertiary amino group.
Synthetic cis-1,4-polyisoprene rubber (hereinafter referred to as IR) has a molecular structure which is similar to that of natural rubber (hereinafter referred to as NR). It is closer to NR in properties than any other synthetic rubber. It has a high degree of workability, and the vulcanized product thereof has excellent physical properties. It is one of general-purpose synthetic rubbers which are used as a substitute for NR in the manufacture of automobile tires, industrial rubber products, rubber footwear, and the like. As IR is a synthetic product, it contains an extremely smaller quantity of impurities and foreign matter than NR. This feature enables reduction in the possibility of occurrence of trouble, such as the production of rejective products, which might be caused by impurities and foreign matter. This is one of the outstanding features of IR which has created a great demand for IR in certain fields of applications.
IR is, however, considerably inferior to NR in green strength. Accordingly, it is likely to cause a cold flow during storage, and various kinds of trouble are likely to occur during the manufacture of rubber products as will hereinafter be pointed out. When unvulcanized materials are put together and molded to form a tire, they are likely to undergo sagging or change in physical property, or the tire cords embedded in the unvulcanized materials are likely to get displaced, resulting in failure to provide uniform tires. The low green strength of IR, however, means its good flowability. It is, therefore, one reason to blend IR into NR despite some reduction in its green strength. Common radial-ply tires, which became using in large amount recently, needs high hardness and modulus, but high and uniform hardness generally was introduced by high hardness of the vulcanized material and flowability of the unvulcanized material. However, the poor flowability of NR, if it is used alone, is often likely to result in various kinds of trouble, including failure to provide a smooth surface or clearly engraved pattern, during calendering, extrusion molding, press vulcanization, or like operation which is performed at a temperature of, say, 80.degree. C. to 150.degree. C.
It has, therefore, been thought desirable to develop rubber having as high a green strength as that of NR in the vicinity of ordinary room temperature, and as high a degree of flowability as that of IR at a relatively high temperature, and which shows excellent physical properties when vulcanized.
Various attempts have hitherto been made to improve the green strength of IR. A typical attempt resides in the addition of maleic anhydride to IR. This method is considered as one of the most effective methods, since modified IR can be produced inexpensively by a relatively simple reaction. The modified IR obtained by this method is, however, inferior to IR in flowability at a high temperature, and the physical properties of its vulcanizate are not satisfactory, though it is comparable to NR in green strength.
Japanese Laid-Open Patent Specification No. 102148/73 proposes the incorporation into synthetic rubber, such as IR, of maleamic acid of the general formula ##STR1## where R stands for a hydrogen atom, or an alkyl, phenyl, chlorophenyl, nitrophenyl, benzyl, formylaminophenyl or cyclohexyl group, or a derivative of maleamide, or maleinhydrazide in order to improve its green strength without lowering the physical properties of its vulcanizate. EXAMPLE 1 shown therein indicates certain improvements in the green strength of synthetic rubber at ordinary room temperature, and the strength and modulus of its vulcanizate, but simultaneously suggests the necessity for improvement in flowability and workability at an elevated temperature, since the breaking strength of the unvulcanizate measured at 80.degree. C. was as high as, or even higher than that of NR which is at least 1.6 times higher than that of IR.
Japanese Laid-Open Patent Specification No. 45198/76 proposes that an amine, such as an alcohol amine or polyethyleneimine having at least three amino groups, be incorporated into modified IR containing maleic anhydride. Although the incorporation of an alcohol amine improves the green strength of the modified IR, it is not so high as that of NR, and the breaking strength and hardness of its vulcanizate are not so high as those of NR. The use of polyethyleneimine does not improve the flowability of the modified IR at an elevated temperature; in other words, its breaking strength at an elevated temperature is not so low as that of IR.
As is obvious from the foregoing description, there has hitherto not been available any modified IR that is satisfactory in all of such aspects as green strength, flowability and workability at an elevated temperature, and the breaking strength and hardness of its vulcanizate. Moreover, the vulcanizates of modified IR obtained by any known method have generally been low in tear strength, even if their breaking strength and hardness are improved.